What I think you miss is that mastery is really just a proxy value for what we really value.
Suppose I'm an artist, with something I feel I want to say, something I want to share with the world. How can I convince you to listen to me, when there's a thousand other people you could listen to? Especially about big issues like what's worthwhile in life.
Well, one way is commitment. Supposedly, Sophie Germain decided as a child to study mathematics because she read the story about Archimedes, who was so obsessed about his circles that he got killed over it. Clearly, Archimedes had found something that really mattered to him, and if Sophie studied it, maybe she would find that meaning too?
So likewise, as an artist I can convince you to take a look by being really dedicated, practicing the craftsmanship side of what I do so that no one can convincingly say they do it better. That doesn't guarantee that what I say is worthwhile to you, but it's at least an indication it was worthwhile to me.
But you know, the converse is not necessarily true. Someone who wasn't the best at the craft and who didn't sacrifice everything for the opportunity to reach you, could still have something wonderful to say.
Sometimes the skill at the craft limits what you're able to say, even if what you wanted to say was great, but I think that's rarely what matters. On a technical level, there's very little that distinguishes, say, the best guitarists in the world.